HRh(dppb)₂, a Powerful Hydride Donor

Abstract

The Rh(I) and Rh(III) hydrides HRh(dppb) 2 and [HRh(dppb) 2 (NCCH 3 )](BF 4 ) 2 (where dppb is 1,2-(bis(diphenylphosphino)benzene) have been prepared, and a structural study of [HRh(dppb) 2 (NCCH 3 )](BF 4 ) 2 has been completed. The latter complex is an octahedral complex with a trans arrangement of the hydride and acetonitrile ligands. A p K a value of 9.4 was measured for this complex by equilibration of [Rh(dppb) 2 ](BF 4 ) with 4-bromoanilinium tetrafluoroborate in acetonitrile. [Rh(dppb) 2 ](BF 4 ) reacts with H 2 in the presence of Pt(dmpp) 2, which acts as a base, to form HRh(dppb) 2 and [HPt(dmpp) 2 ](BF 4 ) (where dmpp = 1,2-bis(dimethylphosphino)propane). An equilibrium constant of 0.42 ± 0.2 was measured for this reaction. Using this equilibrium measurement and a thermodynamic cycle, the hydride donor ability (Δ G ° H -) of HRh(dppb) 2 was determined to be 34 kcal/mol. This value indicates that HRh(diphosphine) 2 complexes are powerful hydride donors. Similarly the p K a value of HRh(dppb) 2 was calculated to be 35 from a thermodynamic cycle that included the potential of the Rh(I/−I) couple ( E 1/2 = −2.02 V vs ferrocene). These results combined with results from the literature suggest the following order of hydricity for five-coordinate, 18-electron hydrides: second row > third row > first row. Similarly an acidity order of second row ≥ first row > third row is deduced.